This invention relates to steam generator structures and particularly to a single plate buckstay stirrup therefor.
Steam generators have rectangular furnaces or gas passes which are defined by walls formed of relatively small diameter tubes. The pressure within the steam generator furnace will be negative during operation of a balanced draft boiler and may have negative pressure excursions under certain operating conditions. The pressure within the steam generator will be positive when a pressure-fired unit is used and in any event may intermittently go positive on any steam generator. These variations in internal pressure cause the walls of the steam generator to buckle inwardly on negative pressure or tend to form a cylinder in the event of positive pressure. In order to maintain structural integrity, it is essential that these walls be maintained essentially flat.
This is accomplished by locating buckstays in the form of I-beams adjacent opposite walls of the furnace so that the buckstays can be attached to the furnace wall to maintain the adjacent wall in a flat condition. Since the furnace walls change in temperature from ambient to temperatures in the order of 300 C., while the buckstays are at ambient temperature at all times, it is essential that arrangements be made to permit appropriate expansion. This is accomplished by carrying the buckstays on the wall to permit the vertical expansion of the walls. The opposing buckstays are tied to one another through connections which are at the boiler wall temperature. This may be in the form of tie plates which are in close contact with the furnace walls and operate as tension members expanding with the furnace wall in connecting the opposing buckstays. The ends of the buckstays are connected by pinned connections at the corners, and the furnace wall is permitted to slide longitudinally of the buckstay with respect to the buckstay as it expands.
If the buckstays were maintained in close contact with the waterwalls, the internal flange of the buckstays would be heated greater then the outside flange thereby causing bowing of the buckstays. Accordingly, it is preferred to locate the buckstays a short distance from the furnace walls with insulation therebetween. Stirrups are used to make this connection between the furnace wall and the buckstay.
U.S. Pat. No. 4,059,075 illustrates such a buckstay stirrup. This buckstay stirrup may be connected to the furnace walls and operates to support the buckstay as well as to prevent relative inward or outward movement between the buckstay and the wall. With negative furnace pressure, the force will normally cause some inward bowing of the buckstay. In order that this does not bind the buckstay in the stirrup, thereby preventing the longitudinal relative expansion, the outer edge of the stirrup which engages the buckstay flange is slightly curved so that only line contact is made in this negative pressure condition. Binding is not significant in the event of positive pressure since this occurs only momentarily and differential expansion need not be permitted at that time.
The buckstay stirrup of U.S. Pat. No. 4,059,075 requires three separate pieces and welding therebetween. A single large steam generator requires about 500 stirrups.